The present invention relates to a wall means intended for apparatus or equipment for continuous separation of a liquid-containing portion from a liquid-containing particle material which is fed continuously through the apparatus in a predetermined direction of feed (A) past the wall means.
Draining liquid through a liquid-containing particle material forming a porous particle bed, e.g. pulp suspensions and wood chips suspended in digester liquor, always results in a consistency gradient occurring in the draining direction of the liquid towards the throttling wall means of a thickener, for instance, for the particle phase such as the fiber or wood chip phase. This consistency gradient increases with increasing drainage velocity and also with increasing time during which drainage occurs without the drainage direction being changed.
Wall means of the type described above in the form of screening elements are known through U.S. Pat. No. 2,998,064 and U.S. Pat. No. 3,385,753 for instance. The inner sides of the bar profiles in the known screening elements have flat surfaces which are directed perpendicular to the central plane of the slots and to the drainage direction of the liquid. However, material collects close to said surfaces, with a relatively high concentration in relation to the surrounding material through which liquid drains in the direction to and through the slots. The high-consistency sections create increased resistance to the surrounding liquid-containing particle material so that its rate of feed is gradually decreased, and the risk of particles becoming caught in the slots is increased, thereby impairing the draining capacity of the screening element. A plug of particle material formed in this or some other way also prevents the liquid-containing particle material from moving linearly forwards. Instead it will be diverted laterally past the plug. Such non-linear movements of the liquid-containing particle material around the plug will also disturb the feeding of the liquid-containing particle material forward in other places in the chamber, particularly upstream of the plug. The result will be uncontrolled feeding of the liquid-containing particle material and a resulting uneven drainage of liquid so that varying consistency gradients will arise at various points within cross sections taken at the level of the plug and upstream or downstream thereof.
The problems mentioned are even more pronounced in screening elements having perforations or short slots.